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Researchers observe waning influenza vaccine effectiveness
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During a recent 4-year period, researchers observed a monthly decrease in protection of the influenza vaccine, including up to 11% per month for one strain.
The decline in vaccine effectiveness (VE) was more pronounced among people who had been vaccinated the previous season, according to Jill M. Ferdinands, PhD, epidemiologist at the CDC, and colleagues.
Ferdinands and colleagues said their findings do not necessarily mean that current vaccine recommendations should be altered.
Currently, the CDC recommends that people aged 6 months and older receive an influenza vaccination by the end of October. Over the past 18 years, influenza seasons typically began in December or early January and epidemics lasted an average of approximately 13 weeks, with some lasting as long as 17 weeks, Ferdinands and colleagues wrote. Last season, it peaked in March — later than usual.
“While the possibility of waning vaccine effectiveness merits further investigation, the current uncertainty in its nature and magnitude make drawing conclusions difficult and suggest that careful consideration of the risks and benefits of delaying vaccination is needed before contemplating changes to current vaccine recommendations,” they wrote in Clinical Infectious Diseases.
Ferdinands and colleagues used data from the U.S. Influenza Vaccine Effectiveness Network to examine the association between VE and time since vaccination among patients aged 9 years and older during four influenza seasons from 2011-2012 through 2014-2015.
VE was strongest shortly after vaccination in all strains.
It declined 7% per month among patients with influenza A (H3N2) — the predominant strain so far this season — with a maximum VE of 35% at 14 days after vaccination, and a VE of zero after 158 days.
Similarly, VE for influenza B viruses was at its greatest — 59% — at 14 days and declined 7% per month. It was 23% at 180 days after vaccination. In patients with A (H1N1)pdm09, VE was 80% at 14 days but declined between 6% and 11% each month. It plateaued at 37% VE at 128 days.
Further, Ferdinands and colleagues observed that a decline in VE with increasing time since vaccination was more pronounced among patients who were vaccinated the previous season.
They noted the possibility that their findings may have other causes, including vaccine drift; confounding by factors such as timing of the vaccination and likelihood of being infected; and bias introduced by changes in the composition of the at-risk population over the course of an influenza season, or by the “leaky” vaccine effect in which patients are infected but show no signs of having influenza.
“With the latter two situations, there may appear to be a decline in vaccine protection because unvaccinated individuals, who have higher risk for infection (assuming vaccine confers at least some benefit), become infected and exit the at-risk population sooner, leaving proportionally more vaccinated individuals in the at-risk population over the course of the season, ultimately leading to vaccinated cases being over-represented later in the season,” Ferdinands and colleagues wrote.
“Bias due to heterogeneity of the at-risk population and the leaky vaccine effect cannot be ruled out as causes of the decline in vaccine effectiveness that we observed; however, models restricted to vaccinees, in which these biases would be expected to be less pronounced, gave similar results.” – by Gerard Gallagher
Disclosure: Please see the full study for a list of all authors’ relevant financial disclosures.
Editor’s note: The story has been updated with the correct age at which the CDC recommends influenza vaccination. The editors regret the error.
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